Processes for making lithographic printing plates by electrophotography are well known. Lithographic printing plates are generally prepared by uniformly charging and imagewise exposing a photoconductive layer of an electrophotographic plate-making material, developing the material in a wet or dry manner to obtain a toner image, fixing the resulting image, and treating the material with a desensitizing solution (etching solution) to render the toner-free non-image areas hydrophilic.
Usually, the electrophotographic plate-making materials described above have a paper support. However, lithographic printing plates prepared using these materials have a printing durability of only about 3,000 copies. The poor durability is due to penetration of water into the paper support. That is, after plate-making, penetration of an etching solution (aqueous solution) takes place upon the desensitizing non-image areas. Further, penetration of dampening water takes place during printing. Thus, the paper support absorbs water and stretches. In an extreme case, the water absorbtion causes delamination between the paper support and the photoconductive layer.
In addition, when using the materials described above, the image quality is such that stable reproducible half tone dots are only about 100 lines/inch. This may be attributed to the moisture content of the support which varies depending upon the temperature and humidity of the atmosphere employed upon exposure. As a result, the electric conductance varies which adversely affects the photographic properties.
Various proposals have been made for removing these problems.
One such proposal is to provide an interlayer between a base paper used as a support and a photoconductive layer. For example, Japanese patent application (OPI) No. 138904/75 describes providing an epoxy resin interlayer; Japanese patent application (OPI) No. 105580/80 describes providing an interlayer comprising an ethylene derivative such as an ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-vinyl acetate copolymer, ethylene-vinyl acetate-vinyl chloride terpolymer or the like; and Japanese patent application (OPI) No. 14804/79 describes providing an interlayer by coating an aqueous polyethylene emulsion of carbon black or graphite and drying it.
However, none of the electrophotographic plate-making materials having the above-described interlayer give rise to lithographic printing plates having excellent printing durability.